Regeneration of ß-Carotene from the Radical Cation by Tyrosine and Tryptophan.

The phenolic amino acid tyrosine (Tyr) was found more efficient in regenerating ß-carotene (ß-Car) from the radical cation (ß-Car(•+)) than tryptophan (Trp) in the presence of base for conditions where the reduction potentials for Trp and Tyr are comparable. Electron transfer from Tyr in 4:1 chloroform/methanol to ß-Car(•+) in the presence of excess base, (CH3)4N(+)OH(-), had a rate close to diffusion control and a second-order rate constant in agreement with the Marcus theory for electron transfer when compared to plant phenols. A maximum of 40% ß-Car was regenerated for ten times excess of Tyr as studied by 532 nm laser flash photolysis followed by transient absorption spectroscopy in the visible and near-infrared regions. The nonregenerated fraction of ß-Car is assigned to secondary degradation processes. For Trp, the rate constant for regeneration of ß-Car(•+) was 1 order of magnitude smaller compared to Tyr and slower than expected from Marcus theory by comparison with plant phenols.